The invention relates to phosphorescent organic luminescent devices. Organic light emitting diodes (OLEDs) have at least one organic layer between two electrodes. The field of OLEDs is usually divided in two broad categories, small molecules (low molecular weight materials), which are processed by thermal evaporation techniques, and solution processable polymer-based OLEDs. Small molecule based OLEDs can have layers that are optimized to perform particular functions, such as charge confinement, electron and hole transport and exciton recombination (that is, bound electron-hole pairs that make up the exciton recombine). The recombination current in any of the layers of a small molecule based OLED can be controlled by incorporating electron and hole blocking layers, by varying the layer thickness and by adjusting dopant concentrations (B. W. D'Andrade and S. R. Forrest, Adv. Mater. 16, 1585, (2004)). Forming small molecule devices using the techniques described by D'Andrade can add to production cost.
Small molecule based OLEDs can include a number of layers. For example, referring to FIG. 1, OLED 100 includes a substrate 110, such as a transparent layer, for example, glass or plastic. A lower electrode, such as an anode 120, is supported by the substrate 110. The anode 120 is formed of a conducting material, such as a transparent material, for example, indium-tin-oxide (ITO). One or more organic layers, such as layers of conjugated polymers or small-molecules, are stacked on the anode 120. Typically, there are at least two organic layers, such as a transport layer and a light emitting layer 150. However, other organic layers can also be formed. In some devices, a hole injection layer 130, a hole transport layer 140, an emitting layer 150, an electron transport layer 160, and an electron injection layer 170 make up the organic layers. An upper electrode, such as cathode 180, sandwiches the organic layers with the lower electrode. The cathode 180 is formed of a metal, such as a low work function metal, e.g., calcium, barium or aluminum, or a salt, such as lithium fluoride, or a combination thereof The anode 120, cathode 180 and organic layers form a light emitting stack 190. To cause the OLED to illuminate, a potential difference is created across a light emitting stack 190. Typically, the OLED is forward biased when the anode is positively biased and the cathode is negatively biased. This causes the anode to force holes toward the emitting layer 150 and the cathode to force electrons toward the emitting layer 150. When the electrons and holes combine, excitons form, which emit light via radiative decay. The electrons and holes combine with each other to form excitons in the recombination zone of the light emitting layer.
Solution processable polymer-based OLEDs can be formed by applying solutions including polymer materials and driving off solvent from the solution to form the organic layers. As few as two organic layers, a hole transport layer and an electron transport layer may be included in the solution processable polymer-based OLED.
OLEDs can have a light emitting layer that is formed from materials that are capable of fluorescing or phosphorescing. Phosphorescent emitters can be more efficient that fluorescent emitters, because phosphorescent materials are able to achieve emission from both singlet and triplet excited states. However, problems with phosphorescent emitters may include forming a device capable of injecting a proper ratio of holes and electrons into the emitting layer, using materials that allow for exciton quenching, including organic materials which have low conductivities and other characteristics that reduce efficiency of the device.